1. Technical Field
The present invention generally relates to a phosphoric acid fuel cell (fuel cell using a phosphoric acid solution as an electrolyte), and particularly to a plate type shift converter used for a phosphoric acid fuel cell to lower a CO concentration.
2. Background Art
Phosphoric acid fuel cells have been developed as a first generation fuel cell and some types of phosphoric acid fuel cells are now practiced.
Referring to FIG. 10 of the accompanying drawings, a typical phosphoric acid fuel cell includes a phosphoric acid solution-soaked electrolyte plate 1, an air electrode 2 and a fuel electrode 3. The electrolyte plate 1 is sandwiched by these electrodes 2 and 3. Further, the electrodes 2 and 3 support catalysts. In this fuel cell, if hydrogen gas (fuel gas FG) is fed to the fuel electrode 3, a following reaction takes place at the fuel electrode side 3: EQU H.sub.2 .fwdarw.2H.sup.+ +2e.sup.-.
As a result, hydrogen emits electrons and becomes an hydrogen ions. The hydrogen ions 2H.sup.+ move through the electrolyte plate 1 and reach the air electrode 2 whereas the electrons 2e.sup.- proceed to the air electrode 2 via an external circuit.
On the other hand, oxygen fed to the air electrode 2 receives the electrons 2e.sup.- and reacts with the hydrogen ions 2H.sup.+ to cause a following reaction: EQU 2e.sup.- +1/20.sub.2 +2H.sup.+ .fwdarw.H.sub.2 O.
Accordingly, water (H.sub.2 O) is produced and a direct current electricity is generated between the air electrode 2 and the fuel electrode 3. This is the power generation by the fuel cell.
One of the conventional power generation systems using the phosphoric acid fuel cell is shown in FIG. 11 of the accompanying drawings.
This power generation system includes a phosphoric acid fuel cell I, a reformer 4, a high temperature shift converter 5, a low temperature shift converter 6, a heat exchanger 8 and a gas-liquid separator 9. The fuel cell I is provided with a cooling portion 7 near the fuel electrode 3. The reformer 4 reforms town gas TG with steam to produce fuel gas. In the high and low temperature shift converters 5 and 6, CO contained in the fuel gas reacts with H.sub.2 O to produce CO.sub.2 and H.sub.2. The heat exchanger 8 is used to recover heat.
In this power generation system, in order to avoid the deterioration of a catalyst held by the fuel electrode 3 of the fuel cell I by CO of the fuel gas, the fuel gas produced by the reformer 4 is introduced to the high and low temperature shift converters 5 and 6 before the fuel gas is fed to the fuel electrode 3. Specifically, the CO concentration of the fuel gas is reduced to 1% or less in the shift converters 5 and 6.
In the shift converters 5 and 6, a following reaction (CO shift reaction) takes place: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2.
FIG. 12 shows a relation between the equilibrium concentration of CO and the temperature when reformed methane gas undergoes the shift reaction. As seen in FIG. 12, the ultimate reaction temperature should be 250.degree. C. or lower, in order to restrain the CO concentration of the fuel gas supplied to the fuel electrode 3 to 1% or less. However, the reaction speed becomes slow at such a temperature. If the reaction speed is slow, all the CO cannot be treated.
To avoid this, as shown in FIG. 11, the fuel gas which has been reformed by the reformer 4 is introduced to a high temperature shift converter 5 of which reaction speed is sufficiently high. The remaining CO is treated by a low temperature shift converter 6 which follows the high temperature shift converter 5, such that the CO concentration is lowered to 1% or less.
The power generation system may include only one shift converter as shown in FIG. 13.
In the system of FIG. 13, the phosphoric acid fuel cell I includes a plurality of fuel cell elements. Each element has an electrolyte plate 1, an air electrode 2 and a fuel electrode 3, and the elements are stacked with separator plates being interposed between the elements. The fuel electrode 3 is provided with a cooling portion 7.
Between the reformer 4 and the shift converter 6, provided is a heat exchanger 10 for the fuel gas. The heat exchanger 10 is used to lower the gas temperature at the shift converter 6 exit to 250.degree. C. or less.
The town gas TG flows through the shift converter 6 from the heat exchanger 10. Then, the town gas TG is introduced into a reforming portion 4a of the reformer 4. At the same time, steam is introduced into the reforming portion 4a from a steam line 11. The town gas is reformed to the fuel gas in the reformer 4. The fuel gas is cooled by the heat exchanger 10 and guided into the shift converter 6. The CO concentration of the fuel gas is reduced to 1% or less in the shift converter 6. After that, the fuel gas is fed to the fuel electrode 3 of the fuel cell I. On the other hand, air A is fed to the air electrode 2. Gases discharged from the fuel electrode 3 (called "anode exhaust gas") are fed to a combustion chamber 4b of the reformer 4. Further, air A is fed to the combustion chamber 4b of the reformer 4. Accordingly, combustible gaseous components among the anode exhaust gas are combusted and the reforming temperature is maintained. Gases from the combustion chamber 4b of the reformer 4 are introduced to a water recovery condenser 12 with gases from the air electrode 2 (called "cathode exhaust gas"). Condensed H.sub.2 O (water) is led to a water tank 13 whereas gases are expelled out of the line.
The water in the water tank 13 is fed to a cooling water circulation line 16 via a water treating device 14 and a water pump 15. A gas-liquid separator 17 is provided on the cooling water circulation line 16. The water in the separator 17 is forced to another heat exchanger 19 by a pump 18 and then part of the water is introduced to the cooling portion 7 of the fuel electrode 3 and the remainder is introduced to the heat exchanger 10 and gas-liquid separator 17. Steam produced in the gas-liquid separator 17 is fed to the reformer 4, as the reforming steam for the reformer 4, via the heat exchanger 10 from a steam line 11.
A coolant circulation line 20 is connected to a heat exchanger 19. A pump 21, a water recovery condenser 12, a waste heat recovery heat exchanger 22 and a cooling tower 23 are provided on the coolant circulation line 20.
An inverter 24 is connected to the fuel electrode 3 and air electrode 2 of the phosphoric acid fuel cell stack I.
An exothermic reaction is caused in the shift converters 5 and 6 of FIGS. 11 and 13 and the CO-containing gases (concentration of CO is between about 2% to about 19%) are shifted in the shift converters. Heat produced upon the exothermic reaction is taken away and the temperature is lowered when the gases are in the shift converters. Accordingly, the gases contain only 1% of CO when they go out of the shift converters. However, since the conventional shift converter has a small heat transfer area, the shift converter should be designed in large dimensions if a sufficient removal of heat produced upon the exothermic reaction is required. Generally, the heat removal is a requisite so that the shift converter cannot be designed compact.
The power generation system using the phosphoric acid fuel cell stack is expected to be used in downtown areas, for example in hotels, hospitals and apartments. However, the power generation system using the phosphoric acid fuel cell stack includes individual units, namely fuel cell stack I, reformer 4, heat exchanger 10 and shift converters 5 and 6 and these units require a large space for their installation. Further, these individual units need complicated pipings between themselves. In addition, a heat loss is quite large in this type of power generation system.